Method for determining loads on a mechanical structure and the resultant damage

ABSTRACT

A method for determination of loads in a mechanical structure and/or of damage or states in the mechanical structure which result from the loads in the mechanical structure. Rotations of a part of the mechanical structure caused by loads/damage in the mechanical structure are measured by a fiber optic rotation sensor that is rigidly connected mechanically to the structural part. The loads/damage/states in the mechanical structure are deduced from the measured rotations.

The invention relates to a method for the determination of loads in amechanical structure and/or of damage or states in the mechanicalstructure which result from the loads in the mechanical structure. Theinvention relates, in particular, to a method for the determination ofbuilding loads and/or building damage resulting from building loads andto a device suitable for this purpose.

Monitoring the structural integrity of buildings during and afterbuilding loads (for example, earthquakes, storms, weight load due tosnow on a flat roof) is an important task. In addition to the need toreduce personal damage, in this context the aspect relating tominimizing downtimes in the utilization of buildings must also be takeninto account. Moreover, towns which are constantly growing in size,which have highly varying terrain properties, lead to the situationwhere the effects of building loads, for example of earthquakes, mayturn out to be very different from one part of a town to another. Thetype and structure of individual buildings within a very narrowlydelimited region also vary very sharply. Consequently, in an extremesituation, it is possible for one building, when subjected to buildingloads, to be seriously damaged, whereas a directly adjacent structureremains virtually intact.

In terms of disaster protection, an extremely difficult situationconsequently arises, since, as regards the optimization and coordinationof rescue measures, there are no simple criteria for prioritizing theaid measures in the event of a disaster in a relatively large municipalarea.

The object on which the invention is based is to specify a method and adevice for the determination of individual building loads and/orbuilding damage, so that a prioritization of the aid measures in theevent of a disaster in a relatively large municipal area is possible.

To achieve this object, the invention provides for the determination ofbuilding loads and/or of building damage resulting from building loads,according to Patent claims 1 and 2. Furthermore, the invention providesmethods for the determination of loads in a mechanical structure and/orof damage or states in the mechanical structure which result from theloads in the mechanical structure, according to Patent claims 7 and 8.Moreover, the invention provides corresponding devices according toPatent claims 9 and 10. Advantageous refinements or developments of theidea of the invention are found in the subclaims.

The method according to the invention for the determination of buildingloads and/or building damage resulting from building loads isdistinguished in that the rotations of a building part which are causedby building loads or building damage are measured via a fibre-opticrotation sensor which is rigidly connected mechanically to the buildingpart, and the building loads/building damage are deduced from themeasured rotations.

In order to make the determination of the building loads/building damagemore precise, a multiplicity of rotation sensors may be employed insteadof a single fibre-optic rotation sensor; thus, rotations of a pluralityof building parts which are caused by building loads/building damage maybe measured via corresponding fibre-optic rotation sensors which arerigidly connected mechanically to the building parts, and the buildingloads/building damage may be deduced from the measured rotations. Inother words, each rotation sensor measures the rotations of thatbuilding part to which it is connected. The overall state of thebuilding can then be deduced from the sum of the determined rotations ofthe individual building parts or the building loads/building damage canbe ascertained individually for each part of the building. Consequently,it is possible, for example, to detect torsions within a building, forexample between two successive storeys of the building.

Each rotation sensor may, if required, be designed individually as auniaxial, biaxial or triaxial rotation sensor, that is to say eachrotation sensor may be designed individually as a sensor which measuresrotations about one, two or three axes of rotation. Thus, for example,particularly critical building parts may be equipped with triaxialrotation sensors, in order to allow precise measurement, whereas, in thecase of uncritical building parts, for example, biaxial or uniaxialrotation sensors may be sufficient.

In one embodiment of the invention, the rotation sensors are fastened toside walls of the building, in such a way that the relative anglesbetween the side walls and the floors or ceilings of the building whichare supported by the side walls can be deduced from the measuredrotations. Such relative angles are a proven measure for the assessmentof building loads/building damage, in particular of earthquake damage.

In addition to the fibre-optic rotation sensors, acceleration sensors orother sensors may be provided on the building, which are rigidlyconnected mechanically to corresponding building parts and via whichtranslations of the building parts which are caused by buildingloads/building damage are measured, building loads/building damage beingdeduced from the measured translations. The additional provision of suchsensors allows a particularly precise determination of buildingloads/building damage, since all six degrees of freedom of movement (tobe precise, three degrees of freedom of translation and three degrees offreedom of rotation) can thereby be measured.

In one embodiment of the method according to the invention, the changein orientation of the building with respect to the axis of rotation ofthe earth is deduced from the measured rotations and/or translations.The change in orientation may be judged to be a reliable measure of thebuilding loads/building damage to be determined, in particular ofearthquake damage.

In generalized terms, the invention provides a method for thedetermination of loads in a mechanical structure and/or of damage orstates in the mechanical structure which result from the loads in themechanical structure. The method is characterized in that the rotationsof a part of the mechanical structure which are caused by loads/damagein the mechanical structure are measured via a fibre-optic rotationsensor which is rigidly connected mechanically to the structural part,and the loads/damage/states in the mechanical structure are deduced fromthe measured rotations.

The invention relates, furthermore, to a method for the determination ofloads in a mechanical structure and/or of damage or states in themechanical structure which result from the loads in the mechanicalstructure. The method is characterized in that the rotations of aplurality of parts of the mechanical structure which are caused byloads/damage in the mechanical structure are measured via correspondingfibre-optic rotation sensors which are rigidly connected mechanically tothe structural parts, and the loads/damage/states in the mechanicalstructure are deduced from the measured rotations.

The term “mechanical structure” is to be understood as meaning, forexample, a building or part of a building (roof), a bridge, a tunnel,the fuselage or wings of an aircraft, part of the ground (for example,the rock of a mountain), a conduit or a traffic route (road, railtracks, railway lines), etc.

The invention provides, furthermore, a device for the determination ofbuilding loads and/or of building damage resulting from building loads.The device is distinguished by

-   -   a fibre-optic rotation sensor which is rigidly connected        mechanically to a building part, and    -   an evaluation apparatus, connected to the rotation sensor, for        determining the building loads/building damage on the basis of        rotations of the building part which have been detected by the        rotation sensor.

The invention relates, furthermore, to a device for the determination ofloads in a mechanical structure and/or of damage or states in themechanical structure which result from the loads in the mechanicalstructure, which device has:

-   -   a fibre-optic rotation sensor which is rigidly connected        mechanically to a part of the structure, and    -   an evaluation apparatus, connected to the rotation sensor, for        determining the loads/damage/states in the structure on the        basis of rotations of the structural part which have been        detected by the rotation sensor.

The statements (embodiments) afforded within the framework of the methodaccording to the invention apply similarly to the devices according tothe invention: thus, the devices according to the invention may alsoconsist of a plurality of fibre-optic rotation sensors; the rotationsensors may be designed as uniaxial, biaxial or triaxial rotationsensors; etc.

The evaluation apparatuses of the individual rotation sensors may benetwork-linkable to further instrumental units, so that rotation datadetermined can be collected in a simple way, so that a rough damagesurvey can be set up even shortly after building loads/building damage.In this case, it is possible both to provide each rotation sensor with aspecific evaluation apparatus and to provide a single central evaluationapparatus which is connected to all the rotation sensors and othersensors. The network-linking of the evaluation apparatuses to furtherinstrumental units or the network-linking of the rotation sensors to acentral evaluation apparatus may take place, for example, via cablelines or via radio connections.

According to the invention, therefore, a sensor based on absoluterotation measurement is employed, which detects the deflection of thebuilding during an earthquake dynamically in three spatial directions bymeans of a specialized fibre-optic gyroscope. In this case, the maximumdeflection and the cumulative deflection of individual storeys or of theentire building are determined in real time and are compared with apredefined building-specific tolerance table which has previously beenanchored in the sensor or in an evaluation apparatus connected to thesensor. To assist immediate measures, the result of this buildingevaluation is indicated, for example, in a simple multi-step colour codeand made available at the same time from an external interface. Thismakes it possible to connect a plurality of such sensors into anoverriding functional unit and to incorporate individual sensors orsensor complexes in a radio network.

The invention is explained in more detail below in exemplary embodiment,with reference to the figures in which:

FIG. 1 shows a diagrammatic illustration of a building part withrotation sensors mounted on it, before and after an earthquake;

FIG. 2 shows a diagrammatic illustration of two building partscontiguous to one another, before and after an earthquake; and

FIG. 3 shows a flowchart of an embodiment the method according to theinvention.

FIG. 4 shows a torsion profile of a building during an earthquake.

Components or component groups corresponding to one another areidentified in the figures by the same reference numerals. In thefollowing description, it is assumed, for the sake of simplicity, thatthe building loads/building damage are earthquake damage.

As has already been indicated, the essential element of the concept is a(preferably) three-component inertial fibre-optic rotation sensor whichis firmly connected to the structural substance. In this way, the sensorparticipates in the movements of the building (or building sections) anddelivers the rate of rotation about three linearly independent spatialdirections (for example, about the “longitudinal axis”, and the“transverse axis” of the building and about the horizontal plane of thelatter) as a sensor signal. These rates of rotation are integrated inthe processor part of the sensor, and the cumulative maximum deflectionwith respect to an inertial reference system is calculated. The maximumdeflection angle between a supporting wall and the concrete ceiling of abuilding is one of the critical variables for assessing the remainingload-bearing capacity of this building part. Too great a deflection of aceiling panel in relation to its support leads to an overstressing ofthe supporting structure and, subsequently, to crack formation which isdetrimental to the load-bearing capacity. In this context, it isunimportant whether this deflection takes place abruptly or in a mannerdistributed over a relatively long period of time. What is decisive forthe durability of the structure is the maximum deflection angle.

FIG. 2 illustrates this relation graphically. Thus, FIG. 2 a) outlines aconcrete ceiling 2 suspended normally on a side-wall part 1, withoutloads due to building inclinations, whilst b) reproduces the situationafter an earthquake. If the angle of inclination (alpha) which hasoccurred due to the earthquake overshoots a particular threshold value,the building may collapse.

As compared with the use of conventional accelerometers, the methodaccording to the invention has the advantage that it is based on theprinciple of absolute rotation measurement and covers a very broaddynamic range. Thus, not only high-frequency deflections are detected,but also inclination variations taking place very slowly in frequencyranges which are no longer accessible by inertia sensors.

Depending on the arrangement of a plurality of sensors in a building,therefore, not only is the inclination of a structure assumed to berigid obtained, but also the differential inclination or torsion betweenindividual storeys and building parts equipped with such sensors.

Accelerometers which function according to the concept of mass inertiamay additionally be employed here. It thus becomes possible, for thefirst time, to acquire a complete sensor for all six degrees of freedomof movement (to be precise, three degrees of freedom of translation andthree degrees of freedom of rotation).

The building sensor 3 according to the invention (which may also containtranslation sensors in addition to a fibre-optic rotation sensor)detects rates of rotation or the integral deflection angle of a buildingor building part under the action of an earthquake. For this purpose, itis to be mounted rigidly on fixed side walls 1 which represent thebuilding behaviour, as shown in FIG. 1: FIG. 1 a) shows(diagrammatically) the situation before an earthquake and FIG. 1 b) thesituation after an earthquake, for one spatial direction (the assumedinclination of the building is in the paper plane; the sensitive axis ofrotation of the building sensor 3 projects out of the paper plane).

In principle, damage to the building may also take place in the otherhorizontal spatial direction, as may also occur in a rotation about thevertical axis (torsional vibrations). A complete sensor triad covers allthe directions of movement. Depending on the structure of the building,however, there may also be directions which are less at risk, forexample on account of higher rigidity, so that individual sensorcomponents may be dispensed with under certain circumstances.

Since rotation sensors detect rotations absolutely on the basis of theSagnac effect, the orientation of a building in relation to the axis ofrotation of the earth can be evaluated automatically before, during orafter an earthquake in real time as a measurement criterion. This makesit possible to determine the change in orientation of a building,without having to rely on local references which could, of course, havechanged due to the action of an earthquake.

FIG. 3 shows an embodiment of a method according to the invention: thesensor 3 measures in continuous sequence the rate of rotation and theangle of rotation (this corresponds to the deflection of the building orbuilding part) and, furthermore, determines the scalar product betweenthe earth rotation vector and the sensor normal (sensitive axis) in astep S1. Under the action of an earthquake or of a high wind load (stormgust), the observation variables yield greater amounts which arecompared by the control electronics with a tolerance-value tableanchored in the program (step S2). This table is coordinatedspecifically with each building. From the comparison between theindividual building tolerances and the instantaneous measurement values,the processor determines the hazard potential due to the external event(step S3). In the simplest instance, this is signalled by a four-stepcolour code: green, yellow, orange and red.

By dynamic radio network-linking of any of these sensors, for example inentire town parts, an area-covering rough damage survey can then be setup, close to real time (smart sensor application). This could beretrieved and utilized, for example, by disaster protection services,for the preparation of priority lists (step S4).

Further aspects of the invention are to be explained in the followingdescription:

Although fibre-optic gyroscopes do not yet have the sensor sensitivityof large ring lasers, nevertheless, because of their small dimensionsand low power consumption, they are suitable both for field use in theregion near a seismic event (aftershock zones, volcanoes, etc.) and forthe instantaneous detection of building states after earthquake actions,but also for the monitoring of static deformations. Combining rotationsensors and seismometers or accelerometers affords the possibility ofdeveloping and utilizing a complete sensor for all six degrees offreedom of movement. An important aspect in this context is to take intoaccount the particular conditions of rotation sensors for measurementfree of local reference symbols, for example the possibility of absoluteorientation of the sensor in real time, and also the self-calibration ofthe sensor by means of the global earth rotation signal.

According to the invention, novel mobile and cost-effective sensors areprovided, which, together with classic seismic sensors, make it possibleto observe ground movements correctly, specifically in all six degreesof freedom of movement: for a complete description of ground movements,not only translations, but also rotations, must be observed with highresolution.

According to the invention, long-term extensive statements on variationand deformations can be made by means of broadened six-componentrecordings (novel measurement methods which can be used in a mobile,cost-effective and rapid way). The additional observation variable“rotational movements” can also be used, above all, in the inversion ofseat parameters (dynamic rotation), and its accuracy can be improved. Inthis case, in particular, the otherwise inseparable coupling ofinclination and translations of the measured signals on the sensor planecan be investigated and corrected by the provision of correspondinglyhigh-resolution rotation measurements. Since the transverse accelerationand the rate of rotation during an earthquake are in phase with andproportional to one another, it is not possible to prevent theseismograph from including a coupling of earthquake-induced inclinationfractions into the sensor signal of the other linearly independentspatial directions in a zone near the earthquake event. Only a sensorwith all six independent degrees of freedom of movement, that is to sayincluding the rotation, can afford an improvement here.

The concept according to the invention in the design of fibre-opticgyroscopes can cost-effectively increase the sensitivity of existingrotation sensors based on glass fibres by up to two orders of magnitude,as compared with the previous prior art, and, when supplemented by abroad-band seismometer, can for the first time make all six degrees offreedom of movement available with sufficient sensitivity in a portableimplementation capable of being used in the field.

The invention concentrates on the utilization and further development ofmodern rotation sensors as a novel and innovative basic technology foruse in early-warning systems. The diverse novel technologicalpossibilities arising from this are to be applied, in particular, to thearea of the monitoring of building structures and the area ofseismology.

After an earthquake, particularly in larger metropolitan areas, a rapidestimation of the damage to important buildings is of centralimportance, particularly in order to optimize the rescue measures. Ifthe co-seismic movements (time profile of the deformations, staticdisplacements and static rotations) could be measured and could beevaluated in terms of critical variables, close to real time, in anindependent sensor, then deformation-induced variations could bedetermined virtually instantaneously, above all for critical buildingsand conduits or traffic routes. According to the invention, acost-effective intelligent 6-C sensor is provided, which can beinstalled in the manner of a “black box” in buildings, bridges, tunnelsor other structures and which records useful data material relating tothe building behaviour during an earthquake or another deforming eventand provides this data material for analysis. According to theinvention, therefore, particular attention is also placed on utilizingthe properties of the rotation sensors as absolute protractors. Thedevices according to the invention are distinguished by a low powerconsumption and are suitable in manufacturing terms for cost-effectivemass production. The installed electronics, by the instantaneousintegration and differentiation of the sensor signals within the sensor,can thus be expected to deliver a hazard or damage estimation, close toreal time, for the respective building. The additional capability of thecombination of such sensor units of independent design into autonomouslocal or regional networks is in this case possible.

The aim, in terms of structural mechanics, is, via suitable sensors, todetermine dynamic excitation as a result of earthquakes or other subsoilmovements, such as vibrations, both as regards amplitudes and as regardsrelevant wavelengths. By coupling geophones or acceleration sensors withrotation sensors as absolute protractors, both characteristic variablesof dynamic excitation can be detected by means of a small number ofmeasurement points. For linear structures, the curvatures caused by theaction can be described from measured rotations. According to theinvention, building on this, the evaluation of the excitation can becarried out via a comparison of the measurement results with theassumptions on which the dynamic design of the structure is based.Furthermore, in addition to the detection of dynamic excitation, theremaining static deformation caused by the action is of interest. Forexample, by lining up a plurality of sensors with one another, theremaining deformation line can be determined via the measured rotations.The possibility of measuring the (quasi-)static and the dynamicinformation by means of cost-effective sensors constitutes aconsiderable advantage, as compared with other measurement methods. Theself-calibration of the sensor and its effect independent of position asregards weight open up a broad field of use also in terms of earlywarning against the consequences of antropogenic actions or criticalaction/resistance combinations, such as, for example, high snow loads onweakened structures, actions due to extreme wind loads, etc. and couldthus allow the use of the sensors even for tasks which, with othermeasurement methods, can be carried out only in a very complicated wayand not in real time. An important area of early-warning systems whichgoes beyond seismic excitation and is covered by the method according tothe invention is the monitoring of subsoil movements, such as occur intunnel building, during earth excavation, due to foundation loads orelse in hill subsidence zones. Consequently, with the aid of sensors,monitoring covers dynamic actions, that is to say the causes ofdeformations or damage, together with the remaining deformations.

Deformation observations in earthquakes and on volcanoes

The problems in determining the static (and dynamic) deformation fromseismometer observations have been known for decades, without asatisfactory stable solution having been found hitherto. The main reasonfor the problems is the additional effects in seismographs which occurdue to rotations and changes in inclination and lead to the situationwhere speed or acceleration seismographs cannot be integrated. GPSinstruments are only partially an improvement here, since (1) thevertical resolution is low and (2) the sensing rate is not sufficientlyfine for the dynamic observations. With cost-effective 6-C seismometerscapable of mobile use, an area-covering observation of dynamic andstatic deformations would be possible. This could, in the long term,also allow improvements in the real-time determination of seatparameters. Similar arguments apply to seismic observations of volcanoesin which sometimes pronounced deformations are to be expected. Theunavoidable coupling of rotational and tilting movements influences thestandard broad-band measurements by means of seismometers and makesmodelling more difficult. The combined data analysis of rotations andtranslations makes it possible, in this area, to have an improvedmapping of seat processes and of the state of the magma chambers.

In one embodiment of the invention, both the dynamic ground/buildinginteraction and the structural dynamics of the building structure are tobe taken into account in determining the loads/damage.

The aim of the invention is to reduce earthquake-induced hazards andrisks. The observations of the entire movement components have beenurged for decades by theoretical seismologists. The most recentdevelopments in sensor technology now seem to allow the design of asuitable measuring instrument with the required accuracy. In thiscontext, it should be noted that measurement technology has a broadpotential for use in both the purely scientific sector and theengineering sector.

The ever-continuing rapid growth of large metropolitan areas inearthquake-endangered regions (for example, Istanbul, Tokyo, Los Angelesand New Mexico) leads to, in the case of a strong earthquake triggeredbeneath them, to tremendous damage and the loss of many human lives. Onaccount of the completely heterogeneous building structures and variablesubsoil properties in these mega-metropolises, it is not possible in ashort time for the respectively responsible disaster management toobtain a survey of the damage situation. The rapid initiation of acoordinated rescue action is consequently seriously impeded, and theexpected result is not the best possible. Autonomous monitoring systems,which co-log a damage event and evaluate, close to real time, accordingto predefined methods and make the result quickly available, could leadto a decisive improvement here. By dynamic weighting according to thefunction of the building and the automatic damage assessment carried outby the sensor, and also other criteria to be laid down (for example,optimization of the travel distance for rescue forces), an effectiveutilization of the critical first 6 hours after a damage event may beenvisaged.

The building sensor concept according to the invention is innovative intwo respects. On the one hand, it is intended to show the way to amonitoring system in which the building behaviour is logged in detailduring the action of external forces (earthquake, extreme wind load,ground subsidences, etc.). In this case, a complete independence oflocal reference systems is achieved via the property of the rotationsensors used as absolute protractors. In this regard, the sensorcorresponds to a “black box”, such as is employed in aircraft. Inaddition to these logging functions, key parameters (for example,maximum deflection) are investigated, during measurement, for theovershooting of building-specific predefined limit values, and, ifpossible, may allow a classification of the damage caused (for example,the degree of risk of collapse) and transfer this to a central location(local disaster protection cell) (use of mains power-independent radiotechnology). This would consequently perform an important step towardsquasi-real-time damage prognosis.

The sensors developed for field use in the region of strong groundmovement have several shortcomings in seismology. Inclinations of theseismometer induce both a sensor signal and the translations actually tobe measured. In near-earthquake zones, the complete movement vectorcannot be reconstructed on account of the absent degrees of freedom ofrotation. Consequently, the measurements are both incomplete and,because of the intermixing of signal sources, faulty. This constitutes aserious impediment to the inversion of earthquake parameters. It is tobe expected that the complete detection of all six degrees of freedom ofmovement and the clear separation of translation and rotation(inclination), along with sufficient sensor sensitivity, can make amarked contribution to improving the seismological models (independencefrom apparatus transfer functions).

Ring lasers are highly sensitive active optical interferometers and,hitherto, the only instruments which can quantitatively determine therotation signals from teleseismic events. Even for use in seismology,where long-term stability is not a major factor, they still require avery high temperature constancy of better than 0.1 degrees per day.Thermally induced expansion or contraction of the optical resonatorwithin the range of a few hundredths of a micrometer cause a drift inthe optical operating frequency which then leads to sudden jumps in thelongitudinal mode index (mode jump). Furthermore, despite all thesimplifications which it has been possible to implement successfully inthe course of the GEOsensor project, ring lasers remain a complexinstallation. This makes them unsuitable for three specificapplications. (1) A short-term transfer of the sensor into the vicinityof the epicentre after a strong earthquake, in order to recordaftershocks in the near field. (2) The detection of the completemovement (all six degrees of freedom) under the influence of strongground movements. (3) As part of a monitoring building sensor system,they are too costly and much too sensitive. On the other hand,fibre-optic rotation sensors, although not having the sensitivity ofring lasers, are nevertheless robust and compact sensors with a markedlyhigher temperature tolerance. Furthermore, they are significantly morecost-effective.

The method according to the invention can utilize theself-calibratability of the system. Since rotational sensors measurerotations absolutely on the basis of the Sagnac effect, the earthrotation signal is also always superposed on the sensor signal. Thissignal may be considered, within the framework of the applicationproposed here, as a constant reference. It is available at any timepoint, that is to say before, during and after an earthquake, as areference independent of the local surroundings and can be used in orderto determine the instantaneous orientation of the sensor in relation tothe axis of rotation of the earth. A comparison before and after anearthquake delivers a change in orientation of the sensor due to theseismic event or to the building deformation. Since the sensor ismounted firmly on a wall for a measurement, this change in orientationcorresponds either to a building inclination or to a deformation or to adisplacement between individual storeys. This signal is determined bythe integration of the measured rate of rotation, and, even after ashort integration time of approximately one minute, a resolution of lessthan 1 degree is achieved. Strictly, this method of determining theorientation of the sensor applies with high resolution in thenorth/south direction only. The resolution is lower in the east/westdirection. The investigation of the optimal sensor implementation(reference system of the sensor) is therefore a specific operatingpoint. After the investigation as regards the utilization of theorientation signal, the utilization of the rotation signal of thebuildings is a major factor. For this purpose, (1) criteria are to beset up between measured angular deflection and stress of the building orvarious materials. (2) The effect of the displacement of individualstoreys in relation to one another is to be quantified (interstoreyrotation) and transmitted to the sensor system.

The output signal of an FOG (Fibre-Optical Gyro) is the phase differencebetween light waves running around a surface in opposite directions. Itis proportional to the rotational speed of the sensor and to the surfacecircumscribed by the light beams. The main advantage of an FOG, ascompared with a ring laser, is that the optical signal can be guided ina glass fibre. In an FOG, the effective surface can be increased,without the dimensions of the sensor having to be varied appreciably. Onthe other hand, fibre-optic gyroscopes differ from ring lasers in thatthey carry out phase measurement, whereas ring lasers determine afrequency difference by interferometry and thus achieve a higherresolution by virtue of the concept adopted. This is a greatdisadvantage in the implementation of an FOG. The signal-to-noisecoefficient (resolution) of an FOG depends not only on the scale factor,but also on the optical power density. For example, for a light sourcewith a wavelength of 1.55 μm, a luminous power of PO=100 μW, a length ofthe glass fibre of 1000 m and a diameter D of the coil of 0.4 m, atheoretical value for the sensitivity of an FOG of 4.125·10-8 rad/s isobtained.

The coil consisting of polarizing optical fibre constitutes an essentialcomponent of the optical path. The fibre length and the geometricdiameter of the coil are incorporated linearly into the scale factor ofthe gyroscope. In principle, therefore, lengthening the coil fibre usedmakes it possible to achieve ever higher accuracies in the measurementof the rate of rotation. At the same time, however, attenuation lossesof the light intensity during passage through the fibre and phase changeoccurring due to disturbing effects, such as, for example, the Shupeeffect or the Kerr effect, limit the implementable length of the fibrecoil. Coils with a length of approximately 1000 m and with a diameter ofup to 300 mm are therefore planned for the prototypes.

FIG. 4 shows a short extract from a measurement of torsion (left-handside), in which a building model (right-hand side) with eccentric massdistribution has been exposed on a vibrating table to an artificialearthquake.

It may be mentioned, in this regard, that earthquake-induced rotationobservations of teleseismic events are compatible with collatedbroad-band translation measurements, and that additional information iscontained in the amplitude ratio.

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 12. A method for the determination of damage or states of amechanical structure which result from a loading of the mechanicalstructure, characterized in that in each case a sensor signal of afiber-optic rotation sensor connected mechanically rigidly at least topart of the mechanical structure is determined before and after theloading; in each case an earth's rotation signal superposed on thesensor signal is determined from the respective sensor signal; on thebasis of the superposed earth's rotation signals determined in eachcase, a respective instantaneous orientation of the fiber-optic rotationsensor with respect to the earth's rotation axis is determined; a changein orientation of the rotation sensor is determined from the respectiveinstantaneous orientations of the fiber-optic rotation sensor before andafter loading, and the damage or states of the mechanical structure aredetermined on the basis of the change in orientation of the rotationsensor.
 13. The method as claimed in claim 12, characterized in that ineach case a further sensor signal of a further fiber-optic rotationsensor connected mechanically rigidly at least to a further part of themechanical structure is determined before and after the loading; in eachcase a further earth's rotation signal superposed on the sensor signalis determined from the respective further sensor signal; on the basis ofthe further superposed earth's rotation signals determined in each case,a respective further instantaneous orientation of the furtherfiber-optic rotation sensor with respect to the earth's rotation axis isdetermined, a further change in orientation of the further rotationsensor is determined from the respective further instantaneousorientations of the further fiber-optic rotation sensor before and afterthe loading, and the damage or states of the mechanical structure aredetermined on the basis of the changes in orientation of the rotationsensor and of the further rotation sensor.
 14. The method as claimed inclaim 12, characterized in that the respective rotation sensors measurerotations about 1, 2 or 3 rotation axes.
 15. The method as claimed inclaim 12, characterized in that the mechanical structure is in the formof a building.
 16. The method as claimed in claim 15, characterized inthat the rotation sensors are fastened to sidewalls of the building insuch a way that the relative angles between the sidewalls and the floorsand ceilings of the building which are supported by the sidewalls can beinferred from the measured rotations.
 17. The method as claimed in claim15, characterized in that the translations of at least one building partwhich are caused by the building loads/building damage are measured viacorresponding acceleration sensors which are connected mechanicallyrigidly to the at least one building part and the buildingloads/building damage are inferred from the measured translations. 18.The method as claimed in claim 17, characterized in that the change inorientation of the building with respect to the earth's rotation axis,which change in orientation is a measure of earthquake damage to bedetermined, is inferred from the measured rotations and translations.19. A device for the determination of damage or states of a mechanicalstructure which result from a loading of the mechanical structure,characterized by a fiber-optic rotation sensor which is connectedmechanically rigidly at least to part of the structure; and anevaluation means which is connected to the rotation sensor and which isdesigned for determining in each case a sensor signal of the fiber-opticrotation sensor before and after the loading; for determining in eachcase an earth's rotation signal superposed on the sensor signal from therespective sensor signal; for determining a respective instantaneousorientations of the fiber-optic rotation sensor before and after theloading, and for determining the damage or states of the mechanicalstructure on the basis of the change in orientation of the rotationsensor.
 20. The device as claimed in claim 19, characterized by afurther fiber-optic rotation sensor which is connected mechanicallyrigidly to a further part of the mechanical structure and, furthermore,is connected to the evaluation means.
 21. The device as claimed in claim20, characterized by in each case an evaluation means, connected to therespective rotation sensor, of an independent wireless data transmissionsystem for the joint determination of the damage or status of thestructure on the basis of rotations or torsions with respect to theearth's rotation axis which are detected by the individual rotationsensors and are jointly analyzed.